1. Field of the Invention
This invention relates generally to a laser-plasma extreme ultraviolet (EUV) radiation source and, more particularly, to a laser-plasma EUV radiation source that provides a stable solid filament target.
2. Discussion of the Related Art
Microelectronic integrated circuits are typically patterned on a substrate by a photolithography process, well known to those skilled in the art, where the circuit elements are defined by a light beam propagating through a mask. As the state of the art of the photolithography process and integrated circuit architecture becomes more developed, the circuit elements become smaller and more closely spaced together. As the circuit elements become smaller, it is necessary to employ photolithography light sources that generate light beams having shorter wavelengths and higher frequencies. In other words, the resolution of the photolithography process increases as the wavelength of the light source decreases to allow smaller integrated circuit elements to be defined. The current trend for photolithography light sources is to develop a system that generates light in the extreme ultraviolet (EUV) or soft X-ray wavelengths (13-14 nm).
Various devices are known in the art to generate EUV radiation. One of the most popular EUV radiation sources is a laser-plasma, gas condensation source that uses a gas, typically Xenon, as a laser plasma target material. Other gases, such as Argon and Krypton, and combinations of gases, are also known for the laser target material. In the known EUV radiation sources based on laser produced plasmas (LPP), the gas is typically cryogenically cooled in a nozzle to a liquid state, and then forced through an orifice or other nozzle opening into a vacuum chamber as a continuous liquid stream or filament. Cryogenically cooled target materials, which are gases at room temperature, are required because they do not condense on the EUV optics, and because they produce minimal byproducts that have to be evacuated by the vacuum chamber. In some designs, the nozzle is agitated so that the target material is emitted from the nozzle as a stream of liquid droplets having a certain diameter (30-100 xcexcm) and a predetermined droplet spacing.
The low temperature of the liquid target material and the low vapor pressure within the vacuum environment cause the target material to quickly freeze. Some designs employ sheets of frozen cryogenic material on a rotating substrate, but this is impractical for production EUV sources because of debris and repetition rate limitations.
The target stream is illuminated by a high-power laser beam, typically from an Nd:YAG laser, that heats the target material to produce a high temperature plasma which emits the EUV radiation. The laser beam is delivered to a target area as laser pulses having a desirable frequency. The laser beam must have a certain intensity at the target area in order to provide enough heat to generate the plasma.
FIG. 1 is a plan view of an EUV radiation source 10 of the type discussed above including a nozzle 12 having a target material chamber 14 that stores a suitable target material, such as Xenon, under pressure. The chamber 14 includes a heat exchanger or condenser that cryogenically cools the target material to a liquid state. The liquid target material is forced through a narrowed throat portion 16 of the nozzle 12 to be emitted as a filament or stream 18 into a vacuum chamber towards a target area 20. The liquid target material will quickly freeze in the vacuum environment to form a solid filament of the target material as it propagates towards the target area 20. The vacuum environment and vapor pressure within the target material will cause the frozen target material to eventually break up into frozen target fragments, depending on the distance that the stream 18 travels.
A laser beam 22 from a laser source 24 is directed towards the target area 20 to vaporize the target material. The heat from the laser beam 22 causes the target material to generate a plasma 30 that radiates EUV radiation 32. The EUV radiation 32 is collected by collector optics 34 and is directed to the circuit (not shown) being patterned. The collector optics 34 can have any shape suitable for the purposes of collecting and directing the radiation 32, such as a parabolic shape. In this design, the laser beam 22 propagates through an opening 36 in the collector optics 34, as shown. Other designs can employ other configurations.
In an alternate design, the throat portion 16 can be vibrated by a suitable device, such as a piezoelectric vibrator, to cause the liquid target material being emitted therefrom to form a stream of droplets. The frequency of the agitation determines the size and spacing of the droplets. If the target stream 18 is a series of droplets, the laser beam 22 is pulsed to impinge every droplet, or every certain number of droplets.
It is desirable that an EUV source has a good conversion efficiency. Conversion efficiency is a measure of the laser beam energy that is converted into recoverable EUV radiation. In order to achieve a good conversion efficiency, the target stream vapor pressure must be minimized because gaseous target material tends to absorb the generated EUV radiation. Further, liquid cryogen delivery systems operating near the gas-liquid phase saturation line of the target fluid""s phase diagram are typically unable to project a stream of target material significant distances before instabilities in the stream cause it to break up or cause droplets to be formed. As a result, the time the stream is in the vacuum chamber prior to stream break-up will be insufficient to allow evaporative cooling to freeze the stream and thereby lower its vapor pressure. Moreover, the distance between the nozzle and the target area must be maximized to keep source heating and condensable source debris to a minimum.
In accordance with the teachings of the present invention, an EUV radiation source is disclosed that creates a stable solid filament target. The source includes a nozzle assembly having a condenser chamber for cryogenically cooling a gaseous target material into a liquid state. The liquid target material is filtered and sent to a holding chamber under pressure. The holding chamber allows entrained gas bubbles in the target material to condense into liquid prior to the filament target being emitted from the nozzle assembly. The target material is forced through a nozzle outlet tube to be emitted from the nozzle assembly into a vacuum chamber as a liquid target stream. A thermal shield is provided around the outlet tube to maintain the liquid target material in the cryogenic state. The liquid target stream freezes in the vacuum chamber and is vaporized by a laser beam from a laser source to generate the EUV radiation.
Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.